1. Field of the Invention
The present invention concerns in general terms a method of allocating radio transmission resources for a mobile telecommunication system, particularly for a telecommunication system of the UTRA-TDD type.
2. Discussion of the Background
FIG. 1 depicts highly schematically the access network in the context of the UMTS, known as UTRAN (standing for UMTS Terrestrial Radio Access Network). The access network effects the connection between the mobile terminal of the user (MS, standing for Mobile Station) and the interconnection network (CN, standing for Core Network). It includes access network controllers (RNC, standing for Radio Network Controller) and base stations (also referred to as B nodes), each RNC controller being able to control a plurality of base stations. The downlinks between the base station and the mobile terminals which come under it are separated from the uplinks by the use of different frequencies, a so-called frequency duplex mode or FDD (standing for Frequency Division Duplex), or by different transmission time ranges, a so-called time duplex mode or TDD (standing for Time Division Duplex).
FIG. 2 depicts schematically a transmission frame in a UTRA-TDD (UMTS Terrestrial Radio Access Network in TDD mode) telecommunication system. The transmission frame has a duration of 10 ms and is divided into 15 transmission time slots denoted TS0 to TS14. The transmission frame is also divided into at least two distinct time ranges (duplex partitioning) allocated respectively to the uplinks (the up arrows) and to the downlinks (the down arrows). The separation between consecutive time ranges is referred to as the switching point. A frame generally has several switching points. In order to reduce interference, the base stations adjacent to a given base station are synchronous with it and have the same switching points. The communications, up or down, within a cell served by a base station are isolated from the communications of an adjacent cell by means of codes known as scrambling codes. The codes relating to adjacent cells are chosen so as to have a low level of intercorrelation in order to minimise inter-cell interference. However, in practice, because notably of the scattering in the delays caused by multipath propagation, the level of intercorrelation is still too high on reception. As a result the data transmitted at a given moment within a cell may be interfered with by data transmitted at the same moment within an adjacent cell. This inter-cell disturbance or interference is variable since it depends on many factors, amongst others the spreading codes, scrambling codes, transmission powers and characteristics of the different transmission channels of the various users. However, the interference level varies little within a time slot from one frame to another if the connections and the allocation of the radio resources do not change.
In order to combat inter-cell interference in the UTRA TDD system, a dynamic allocation of the time slots or DCA (Dynamic Channel Allocation) is provided, consisting of dynamically allocating time slots and access codes to the different communications. It has been proposed to divide this dynamic allocation into a slow allocation (Slow DCA) and a fast allocation (Fast DCA). In the context of this proposal, the slow allocation is supervised by the RNC and aims at settling conflicts in resources between adjacent base stations. The rapid allocation, for its part, is dealt with by the base station and its function is the dynamic allocation, to the different communications, of the resources allocated by the slow allocation. To do this, the base station regularly transmits a measuring request to the mobile terminals. On such a request, a mobile terminal effects a measurement of the interference level and transmits the result thereof to the base station. The base station then allocates the available resources according to these results. This allocation being carried out at a sufficiently high frequency, to make it possible to follow the rapid variations in the interference levels, the result is a high signalling rate (3 messages per allocation cycle), which burdens the radio resources of the system.
The problem at the basis of the invention is to allow a dynamic allocation of the radio resources which requires only a little signalling.